1. Field of the Invention:
The present invention relates to optical interferometers. In particular, the present invention is an apparatus and method for converting a Michelson interferometer into a double-pass interferometer.
2. Description of the Prior Art:
Interferometers are well known optical devices commonly used to measure unknown lengths in terms of known wavelengths of light. Ultraprecision positioning systems, precision length measurement, thickness detection and Fourier spectroscopy are but a few of the many applications in which interferometers are found.
In a typical amplitude-splitting interferometer, an incident beam of radiation is impinged upon a beam splitter and divided into first and second beams. These two beams are each propagated through a separate optical path and modulated before being recombined. The recombined beams form an output beam of radiation which is impinged upon a detector. An interference pattern, or interferogram, within the output beam is indicative of the difference in length between the first and second optical paths. The detector produces an electrical signal as a function of the interference pattern.
Mirrors, corner cube prisms or similar optical reflection means are typically used to direct the first and second beams through their separate optical paths. At least one of these reflectors is movable to permit the length of one of the optical paths to be adjusted. The incident beam of radiation is thereby modulated and the interference pattern produced within the output beam of radiation.
In practice, the moving reflector is cyclically moved in a linear direction and at a constant velocity between two end positions. The distance separating the two end positions is known as the stroke length. Width of the side lobes in the interference pattern, and therefore resolution, is proportional to the stroke length. The interference pattern will be centered at a frequency proportional to the velocity, and therefore frequency, with which the moving reflector is cycled between the two end positions.
Various types of drive systems are used to cycle the moving reflector between its end positions. At the optical frequencies in which an interferometer operates system tolerances are critical. The reflector must be moved at an extremely constant velocity with virtually no vibrations. Variations in either of these parameters cause the interference pattern to be shifted from its center frequency. It becomes increasingly difficult to design a drive system meeting these constraints as the stroke length over which the reflector must be driven is increased. As previously mentioned, system resolution increases as stroke length increases. System design therefore involves engineering tradeoffs between velocity and vibration levels on the one hand, and resolution on the other.
It is known that the resolution of an interferometer can be doubled by redirecting the output beam of radiation through the interferometer a second time. The incident beam of radiation is thereby twice modulated with the width of the interference pattern side lobes being doubled. This increase in resolution is obtained without increasing the distance over which the moving reflector is cycled and, therefore, without the added expense and lower tolerances of a drive system needed to implement this change.
One such double-pass interferometer is disclosed by S. J. Bennett in Optical Communications, Vol. 4, No. 6, pp. 428-430, Mar. 1972. Incident radiation is impinged upon a polarizing beam-splitter whereby the reflected and transmitted beams are orthogonally polarized and propagated along separate optical paths. A quarter-wave plate is positioned in each optical path. Each beam therefore passes through a quarter-wave plate before being reflected by one of the interferometer mirrors and passing through the quarter-wave plate a second time. The plane of polarization of each beam is therefore rotated through 90.degree. before returning to the beam splitter. Both beams then enter the cube-corner reflector since the beam that was first reflected is now transmitted through the beam splitter, while the beam that was first transmitted is now reflected. The retro-reflected beams emerging from the cube-corner relfector are returned to the beam splitter which for a second time directs them through the separate optical paths. Having been rotated through another 90.degree. by two more passes through the quarter-wave plates, the two beams impinge upon the beam splitter and leave the interferometer parallel to the incident beam. The interference pattern in this double-passed output radiation can then be detected.
The Pardue et al. U.S. Pat. No. 4,334,778 discloses a dual surface interferometer which operates on a principle similar to that disclosed in the Bennett article. Light beams of two different frequencies are orthogonally polarized and directed through the interferometer. The Doppler frequency shift in one of the beams corresponds to the direction and velocity of the relative displacement of the opposed reflecting surfaces.
The Williams U.S. Pat. No. 3,109,049 discloses an interferometer in which incident light is split and twice passed through two optical paths before being recombined. Each optical path includes a pair of light reflecting elements which reflect light back on a path parallel to the incident path, but displaced therefrom.
U.S. Pat. No. 3,419,331 discloses a single and double beam interferometer. The single beam interferometer is sensitive to cosine errors in tracking and is used to measure the distance which the movable reflector has moved. The dual beam interferometer, although insensitive to cosine errors in tracking, has twice the sensitivity of the single beam. The advantages of both the single and double beam interferometer are combined on a single optical block.
U.S. Pat. No. 3,788,746 discloses an optical dilatometer, a device for measuring the linear coefficient of expansion of a specimen block. Information is obtained from the interference pattern between a reference light beam and a light beam which has twice traversed the distance to the surface whose motion is being monitored. The disclosed arrangement eliminates the need for a precisely aligned output beam splitter at the point where the interference pattern is formed.
U.S. Pat. No. 3,976,379 also discloses an interferometer. The apparatus causes two light beams which are to be interfered with each other to travel along a common path as far as possible. Even when the interferometer is slightly misaligned for various reasons, the light paths are prevented from being changed in a path difference, thereby obtaining stabilized inteference fringes.
The increased sensitivity and other advantages of double-passed interferometers make these optical devices well suited for a wide variety of applications. It is clear from the prior art, however, that the optical systems required to implement such interferometers are relatively complicated. What is needed is a double-pass interferometer which requires few optical elements and is simple and inexpensive to construct. It would be particularly advantageous if the double-pass interferometer could be constructed by retrofitting commonly used and currently existing single-pass interferometers. Furthermore, the interferometer should make full use of the maximum throughput of the interferometer geometry.